(a) Field of the Invention
The present invention concerns a method of fabricating semiconductor devices by relying on the so-called dry process which is one of the semiconductor device fabricating methods, and more particularly it concerns the method of applying photochemical reaction to the dry process, and also an apparatus therefor.
(b) Description of the Prior Art
For the fabrication of various kinds of electronic devices such as semiconductor devices represented by, for example, transistors and integrated circuits (IC's), there are being adopted fabricating techniques of progressively ascending levels to meet the ever-advancing requirements for realizing higher levels of performance and greater miniaturization of the devices. In these devices, the size of the IC-constituent devices, the intervals between these devices and the diameters of the lead wires formed on integrated circuits have become calibrated to measure by the order of micrometer. Thus, the dimension of the device in the lateral direction is presently limited for a tolerance or error of only about .+-.0.1 micrometer. With respect to the vertical direction, there prevails the requirement for the formation of very thin films having the thickness of about several hundred .ANG.. Depending on the cases, there is the need to provide a multi-layer structure in which these thin films are stacked one upon another into several laminated layers.
For the reasons mentioned above, there has been the constant requirement to develop a very high degree of, i.e. high precision, technique or method in the process of depositing or etching various kinds of such thin films having different functions relative to each other.
The techniques of deposition and of etching which have recently become practiced or have become important of late satisfying the above-mentioned requirements are called the "dry process" in a broad sense of the words in the field of semiconductors.
The mention of the terms "in a broad sense of the words" hereinabove is based on the following considerations.
The technique which is called "photolithography" employed in the field of art of semiconductors points to the art of selectively etching such a film as SiO.sub.2 or Si.sub.3 N.sub.4 formed on the surface of the semiconductor by the use of a photoresist and a chemical etchant containing, for example, fluoric acid (HF). This is an important technique which is being used currently in the process of fabricating semiconductor devices. It has been very difficult, however, to limit the error or tolerance of the dimension accuracy after etching to the above-mentioned level of about .+-.0.1 .mu.m. Accordingly, as a high-precision etching process which can substitute the abovesaid technique, there has been put to practice the so-called sputtering process (including DC sputtering, RF sputtering, microwave sputtering, reactive sputtering, and gas plasma sputtering) which typically is arranged so that a substrate to be etched is placed in a vacuum chamber, and under a gaseous atomosphere produced by introducing an inert gas such as Argon and a reacting gas such as carbon tetrachloride (CCl.sub.4), either a DC voltage or a radio frequency voltage is applied across the electrodes to cause a glow discharge to thereby etch a required site or sites of the substrate. Other than those mentioned above, there has been started to be used also an ion etching technique using an ion beam. The etching mechanism of this latter technique may be regarded to be basically identical with that of sputtering.
In the field of semiconductors, the former-mentioned etching process using a chemical etchant is called the "wet process", in contrast to the latter etching process using abovesaid sputtering techniques, ion beam technique or discharge process in the field of "discharge chemistry" which is customarily called the "dry etching process" or simply the "dry process".
This "discharge chemistry" will be briefed hereunder by taking up an example of the abovesaid sputtering techniques. Into a vacuum chamber containing two opposing electrodes is charged, for example, argon gas (Ar), and a DC voltage is applied across these electrodes to produce a glow discharge. Whereupon, Ar gas is ionized to become Ar.sup.+ which collides against the substrate to drive out the atoms or molecules of the substrate. This process represents the "etching". Instead of using Ar gas, there may be charged such a gas as will cause chemical reaction with the atoms of the substrate. By so doing, there can be performed various processes such as deposition and etching.
However, the technical term "dry process" would be more suitable when it is considered in a broader concept than limiting it only to the use for specific types of etching mentioned above. The term "dry process" used in the present invention indeed represents the abovesaid broader sense. This is because of the consideration that the sputtering method (which, in practice, is called either the reactive sputtering or plasma CVD technique) is used as the art of forming, by deposition, a thin film of such a substance as amorphous Si, polycrystalline Si, SiO.sub.2, Si.sub.3 O.sub.4 or TaN with good precision (i.e. elaborately controlling the thickness as well as the film quality or condition). It should be noted here that the methods of forming a thin film by the sputtering technique or by the vacuum deposition technique are called, in general, the Physical Vapor Deposition (PVD) in contrast to the Chemical Vapor Deposition (CVD). It should be noted also that, for example, the vapor epitaxial growth which is one type of the CVD technique is such that a thermal energy is applied to a reacting gas to cause deposition by virtue of hydrogen reduction or pyrolysis. In contrast thereto, such method as the abovesaid plasma CVD technique is of the mechanism that the discharge energy (electric energy) such as by glow discharge is applied to the reacting gas, and the deposition is conducted under the conditions common to the ordinary sputtering technique and the CVD technique. The deposition mechanism also is not limited to one kind, but combinations of various mechanisms would be necessary for the formation or deposition of a thin film having a high level of functional characteristics.
Viewing this way, it will be noted that not only the ordinary CVD technique but also the deposition process which relies on the decompressed CVD technique intended to improve the uniformity of the thickness of the produced film by elongating the mean free path of the reacting gas may be likewise included in the dry process.
From the above-stated sense, the concept of "dry process" is considered to be applicable to the whole phenomena which would arise between the objective (article to be processed) and the gas-phase material, regardless of whether the process is intended for deposition or etching. Currently, however, the dry process which is capable of forming a thin film whose thickness is controlled efficiently with good precision, or which is capable of performing an etching which realizes the demensional precision with an error of the micrometric order is typically represented, as the main stream of the art, by the specific dry process arranged that a gas is introduced into a chamber in which a glow discharge is developed to render the gas-phase material to the activated state by the discharge energy (electric energy) thus produced, to accelerate the progress of the growth (deposition) of a film. In either the etching or the deposition process, the substrate (hereinafter will be referred to as the objective) on which these processes are conducted is placed in a sealed chamber, and the interior thereof is evacuated, followed by the introduction of a required gas thereinto, and electric power is applied across the electrodes housed in the chamber to develop a glow discharge. Even if the electrodes are set outside the chamber which is made of an insulating material, there will inevitably exist high energy particles in the discharge space.
In such an instance, the objective is placed either on the electrode, or in the vicinity of the electrode, or at a site relatively away from the electrode. In other words, the objective is placed in the region wherein there is developed an intensive glow discharge (i.e. the discharge region), or in a region adjacent to the discharge region but no distinct glow discharge phenomenon is produced (i.e. the non-discharge region). In this latter instance also, the circumstance within the chamber is such that there is hardly any difference in the gas pressure in the discharge region as compared to the region in which the objective is placed. Regardless of in whichever region the objective is to be placed, the material produced in the discharge region (which material, in general, consists of either gas-phase particles which provide the deposition layer or gas-phase particles which serve as the material for etching the objective) is supplied onto the objective.
When a glow discharge is developed by the introduction of a gas, those atoms and molecules which are contained in the charged gas are subjected to a discharge energy to be rendered to the state of having a higher energy, i.e. the activated state. As a result, there are developed in the gas phase not only an increase in the mere kinetic energy of atoms and molecules, but also such complicated reactions as chemical reaction including ionization, decomposition and synthesis, and also polymerization. For this reason, the electricity-charged particles such as electrons and ions are produced in considerably large amounts in the discharge region, and they will acquire a large kinetic energy by being subjected to an electric energy imparted by the glow discharge, i.e. they will acquire an increased velocity. These particles which have acquired an increased velocity will collide against neutral particles such as Ar to ionize them or impart a kinetic energy to them. This means that not only those particles (molecules, atoms, ions, electrons, etc.) which are necessary for the deposition onto or the etching of the objective, but also those particles which are not necessary for these purposes will also be supplied to the objective in either the ionized state from the electrical point of view or the neutral state and with a considerably high kinetic energy. The directions in which they are supplied to the objective are random in general. In certain cases, however, for example, in order to enhance the deposition rate, a magnet is placed in the vessel or the chamber, to uniformalize the orientation of supply of these particles with the aid of the magnetic field produced by the magnet, i.e. giving orientation of movement to the particles, in a certain type of dry process.
Any way, when gas-phase particles having a high kinetic energy as stated above are supplied to the objective, it often happens that the surface of the objective is damaged due to the collision thereagainst of these particles. This damage includes the development of such defects as lattice dislocations, clusterings, strains, etc. in the surface of the objective, aggravating the electric characteristics of the device or devices contained in the objective.
As discussed above, the deposition process and the etching process which are collectively called the dry process utilizing glow discharge is difficult to avoid the drawback, in the conventional art, of damaging the surface of the objective in spite of the fact that this dry process represents a high level of technique which is intended to efficiently control the dimension such as thickness and width of the objective with good accuracy.
Moreover, the value of the energy which is supplied to the gas by glow discharge is averagedly large. However, since the values of energy can range widely, the atoms and molecules contained in the gas would be activated in miscellaneous ways, causing various kinds of physical or chemical reactions to take place. Thus, it will be noted that no particular selected activation necessary for only the desired deposition or etching purpose is carried out in the conventional art.
In case, for example, it is intended to effect the deposition of amorphous silicon (a-Si) by relying on the plasma CVD technique using a gas containing SiH.sub.4, the resulting a-Si film thus formed will be found to contain not only a-Si alone, but also various types of Si.sub.x H.sub.y substances such as Si polycrystals or SiH.sub.4. As will be appreciated from this phenomenon also, the process concurrently has such drawback as represented by the reactions which are not in line with the intended purpose, or by other undesirable reactions. Also, there may occur an instance wherein, although the dry process therein is intended only to the etching of an objective, the performance is such that not only the etching itself is done, but also, apart from that, irrelevant deposition would also take place at the same time.
Thus, there may be considered a method which insures that, among various kinds of atoms and molecules which have been imparted various types of conditions as a result of the activation in the discharge region, only those specfic particles which meet the intended purpose are selectively supplied onto the objective. Such a method, however, would inevitably lead to a very costly large-scale apparatus, and in addition, would give rise to the difficulty to select specific kind of particles with a good efficiency.
As the method of improving these drawbacks and problems of the prior art mentioned above, there have been proposed methods to promote the dry process by externally impinging light rays into the chamber or vessel in which the dry process is to be carried out.
One of such prior methods is designed to place an objective in the chamber in a region located adjacent to the discharge region, and to cause the beam of light rays to impinge onto the discharge region to thereby activate the gas which is charged in the chamber. This proposed prior art requires that the gas pressure is set low to develop a glow discharge, but this leads to a poor efficiency of activation of particles. Moreover, as discussed above, in this method also, particles having a large kinetic energy will collide against the objective, and damage the latter.
Another priorly proposed method is to place an objective in the discharge region of the chamber in which the charged gas is activated, and a beam of light rays are caused to impinge thereonto. It should be noted here that this prior art technique involves the problems that the gas is activated not only by the light rays incident thereonto, but also by the glow discharge as well, so that there will occur not only the aimed reaction but also those reactions which are not in line with the aimed purpose. Moreover, the surface of the objective would become contaminated by those products of such reactions that depart away from the aimed purpose.
Photochemical reaction process per se occurs selectively in many cases. This means that it is possible to develop a reaction selectively. Therefore, such a selective process acts powerfully to realize a clean process.